Shaped articles are predominantly produced by an injection molding process. In the injection molding process a visco-elastic mixture is forced into a mold, heated, optionally under pressure, and then removed from the mold.
In the injection molding cycle, a rubber mixture passes through a pressure profile up to filling of the mold.
During the injection phase, in the region of the capillaries, a high pressure is rapidly built up and the rubber mixture is subjected to a high shear gradient. At the injection nozzle there is a marked release of pressure with the risk that low molecular weight constituents of the rubber mixture can cause “flash vaporization”. During the of filling of the mold the pressure rises again slightly until filling of the mold is complete, it is to be assumed that additives which are concentrated on the surface of the mixing front via demixing processes are no longer taken up by the mixture and can thus cause surface defects up to a lack of welding of the mixture.
The thermodynamic explanation for this is based on a shear field, under the influence of which an entropy gradient develops in the polymer which generates a diffusion flow from the outside to the center of the flow, leading to concentration of low molecular weight constituents on the outer layer of the flow.
Chemicals, which as a result of this process moreover reduce the friction of the rubber mixture on a capillary wall (i.e. cause wall slippage) are conventional processing active compounds such as, are widely employed in the rubber industry.
In order to achieve the highest possible production cycle, the mold, as a rule a metal mold, should not be contaminated by residues of the rubber mixture. If the finished shaped articles can be removed from the mold only with a layer being left behind, mold contamination is referred to. Even small residual amounts add up after several cycles to layers, which can no longer be tolerated.
So-called mold release aids are therefore often added to the rubber mixtures. However, these can in their turn again lead to undesirable deposits.